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Subject[PATCH 2/3] mm, compaction: raise compaction priority after it withdrawns
Date
From: Vlastimil Babka <vbabka@suse.cz>

Mike Kravetz reports that "hugetlb allocations could stall for minutes or hours
when should_compact_retry() would return true more often then it should.
Specifically, this was in the case where compact_result was COMPACT_DEFERRED
and COMPACT_PARTIAL_SKIPPED and no progress was being made."

The problem is that the compaction_withdrawn() test in should_compact_retry()
includes compaction outcomes that are only possible on low compaction priority,
and results in a retry without increasing the priority. This may result in
furter reclaim, and more incomplete compaction attempts.

With this patch, compaction priority is raised when possible, or
should_compact_retry() returns false.

The COMPACT_SKIPPED result doesn't really fit together with the other outcomes
in compaction_withdrawn(), as that's a result caused by insufficient order-0
pages, not due to low compaction priority. With this patch, it is moved to
a new compaction_needs_reclaim() function, and for that outcome we keep the
current logic of retrying if it looks like reclaim will be able to help.

Reported-by: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Tested-by: Mike Kravetz <mike.kravetz@oracle.com>
---
include/linux/compaction.h | 22 +++++++++++++++++-----
mm/page_alloc.c | 16 ++++++++++++----
2 files changed, 29 insertions(+), 9 deletions(-)

diff --git a/include/linux/compaction.h b/include/linux/compaction.h
index 9569e7c786d3..4b898cdbdf05 100644
--- a/include/linux/compaction.h
+++ b/include/linux/compaction.h
@@ -129,11 +129,8 @@ static inline bool compaction_failed(enum compact_result result)
return false;
}

-/*
- * Compaction has backed off for some reason. It might be throttling or
- * lock contention. Retrying is still worthwhile.
- */
-static inline bool compaction_withdrawn(enum compact_result result)
+/* Compaction needs reclaim to be performed first, so it can continue. */
+static inline bool compaction_needs_reclaim(enum compact_result result)
{
/*
* Compaction backed off due to watermark checks for order-0
@@ -142,6 +139,16 @@ static inline bool compaction_withdrawn(enum compact_result result)
if (result == COMPACT_SKIPPED)
return true;

+ return false;
+}
+
+/*
+ * Compaction has backed off for some reason after doing some work or none
+ * at all. It might be throttling or lock contention. Retrying might be still
+ * worthwhile, but with a higher priority if allowed.
+ */
+static inline bool compaction_withdrawn(enum compact_result result)
+{
/*
* If compaction is deferred for high-order allocations, it is
* because sync compaction recently failed. If this is the case
@@ -207,6 +214,11 @@ static inline bool compaction_failed(enum compact_result result)
return false;
}

+static inline bool compaction_needs_reclaim(enum compact_result result)
+{
+ return false;
+}
+
static inline bool compaction_withdrawn(enum compact_result result)
{
return true;
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index d3bb601c461b..af29c05e23aa 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3965,15 +3965,23 @@ should_compact_retry(struct alloc_context *ac, int order, int alloc_flags,
if (compaction_failed(compact_result))
goto check_priority;

+ /*
+ * compaction was skipped because there are not enough order-0 pages
+ * to work with, so we retry only if it looks like reclaim can help.
+ */
+ if (compaction_needs_reclaim(compact_result)) {
+ ret = compaction_zonelist_suitable(ac, order, alloc_flags);
+ goto out;
+ }
+
/*
* make sure the compaction wasn't deferred or didn't bail out early
* due to locks contention before we declare that we should give up.
- * But do not retry if the given zonelist is not suitable for
- * compaction.
+ * But the next retry should use a higher priority if allowed, so
+ * we don't just keep bailing out endlessly.
*/
if (compaction_withdrawn(compact_result)) {
- ret = compaction_zonelist_suitable(ac, order, alloc_flags);
- goto out;
+ goto check_priority;
}

/*
--
2.20.1
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